Sera from ten rabbits of each sex of fifteen inbred and cincipient inbred strains of rabbits will be analyzed for specific complement activity. The assay system will use mouse lymphocytes and tumor cells (Serum I-A) as targets for complement-mediated lymphocytotoxicity by mouse alloantisera. Analysis will focus on determining which strain(s) of rabbits have an inherently high level of serum complement with concomitantly low nonspecific cytolytic activity.